Manchester Memoirs, Vol. xlvi. (1901), No. 5. 3 



several gaseous mixtures, have been most carefully studied 

 by M. Berthelot in France, and H. B. Dixon in this 

 country. 



The all important condition for a gauge destined to 

 record the rise and fall of pressure caused by an explosion 

 is that its time period should be as small as possible. If 

 A represent the force required to produce unit deflection 

 of the vibrating system, W the weight of the moving 

 parts, the time period will be 



I W 



We have therefore two variables at our disposal, nameh-, 

 the weight of the moving parts and the controlling force. 

 The former must be made a minimum, the latter a 

 maximum. 



In most instruments where a short period is desirable, 

 the strains to which the parts are subjected are very 

 small, and the desired result is obtained by decreasing the 

 size of all moving parts, and using, wherever possible, 

 materials of low density. This method is employed in 

 the case of all oscillographs, telegraph recorders, phono- 

 graph receivers, galvanometers, etc. 



In the present case, the instrument having to withstand 

 pressures of ten or twenty thousand pounds per square 

 inch applied with extreme suddenness, strength becomes 

 a condition of vital importance, and steel is the only 

 material which will withstand the strain. We cannot, 

 therefore, use materials of small density, neither can we 

 reduce the dimensions of the moving parts below a certain 

 limit. 



It is thus evident that we must have recourse to the 

 second variable factor to secure the short time period 

 which is necessary. As we have seen above, the control- 

 ling force brought into play per unit length of motion 

 must be as great as possible. In other words, we must 

 use the stiffest spring we can obtain. 



